This invention relates generally to air conditioning systems, and more particularly to controls therefor.
Air conditioning systems are employed to discharge conditioned air into a zone or room requiring conditioned air for the comfort of the occupants thereof. The conditioned air compensates for heat developed in the room from, inter alia, lights, electric machines, occupants, and solar heat developed via radiation and conduction. Typically, in installations such as office buildings, schools, and other multi-room buildings, air is conditioned at a central station and supplied to air discharge or distribution terminals provided in each of the rooms via one or more supply air ducts. Air is then returned to the central station for reconditioning via return air ducts.
In many applications, it is desirable to maintain the quantity of air directed into a room independent of changes in the supply air pressure. For this reason, room discharge terminals of air conditioning systems are generally provided with a valve or valve means for restricting the flow of conditioned air into the room, with the valve or valve means controlled by a signal indicative of the supply air pressure. The valve or valve means is automatically adjusted in response to changes in the supply air pressure so that the amount of air flowing through the valve or valve means is kept relatively independent of fluctuations in the supply air pressure. In many of these same applications, it is desirable, in contrast, to reduce the quantity of conditioned air supplied to the room in response to variations in the air temperature of the room. Accordingly, many room air discharge terminals also include a thermostat or thermostatic control means for sensing the temperature of the air within the room and modifying the control signal supplied to the valve means of the discharge terminal to reduce the quantity of air delivered therefrom as the air temperature of the room approaches a desired level.
Usually, during the evening hours, holidays, or weekends, the cooling load in many rooms of multi-room buildings becomes almost negligible due to elimination of almost all the heat producing elements. However, even those air conditioning systems utilizing room discharge terminals with thermostat controls generally continue to discharge a minimum amount of conditioned air into the rooms of the building, lowering the air temperature of the rooms to a level which may be uncomfortably cool for the occupants initially entering such rooms immediately following the low load period.
Even if the air conditioning system is shut down during a low load period, thus eliminating the flow of conditioned air into the rooms, overcooling of various rooms may still occur via transmission of heat to the outdoors through the walls and windows of the building.
To compensate for such overcooling, many central air conditioning systems of the general type heretofore discussed supply relatively warm air to the rooms of the building shortly before the occupants thereof are due to arrive. The normal thermostatic controls of the room air discharge terminals, sensing the very cool air within the rooms, tend to severely restrict the amount of warm air directed into the rooms. During this warm-up period, such a restriction is generally undesirable and, conventionally, discharge terminals serving those rooms which need warm-up air are each provided with an override control, often referred to as a "warm-up switch", which senses the flow of relatively warm air in the supply air duct and prevents the normal thermostatic control from restricting the flow of this warm air. Thus, the temperature level in the rooms may be readily and rapidly increased to a satisfactory level before the occupants thereof arrive, eliminating occupant dissatisfaction due to excessively low temperature levels.
For one reason or another, however, some rooms of a building may not need a warm-up period. For example, often electrically powered equipment such as computers, photocopiers, and typewriters are concentrated in a few rooms of a building and are almost constantly operated. With the nearly continuous operation of this heat producing equipment, these rooms seldom, if ever, become overcooled. During a warm-up period, the normal thermostatic controls of the discharge terminals serving these rooms, sensing the relatively warm air within the rooms, tend to allow a comparatively large amount of air into the rooms. This, of course, further warms the rooms and, as the rooms warm, the thermostatic controls tend to allow even more air into the rooms. Directing warm-up air to the rooms which do not need it is a waste of the warm air and, in fact, may cause discomfort to the occupants thereof and reduce the efficiency of the complex machinery therein.